Determining the location of the tip of an electronic stylus

ABSTRACT

An electronic stylus system includes an electronic stylus and base receiving unit. The electronic stylus includes a first ultrasonic transmitter, a second ultrasonic transmitter, an electromagnetic transmitter, and a writing tip. The base receiver unit includes a first ultrasonic receiver, a second ultrasonic receiver, and an electromagnetic receiver. The ultrasonic receivers of the base unit are operable to receive signals transmitted by the ultrasonic transmitters of the electronic stylus. Similarly, the electromagnetic receiver of the base unit is operable to receive signals transmitted by the electromagnetic transmitter of the stylus. The location of the tip of the electronic stylus relative to a given reference point is determined using the locations of two ultrasonic transmitters relative to the two ultrasonic receivers.

TECHNICAL FIELD

The field of the invention relates generally to electronic styli, andmore particularly to systems and methods for accurately locating andrecording the position of the tip of an electronic stylus while theelectronic stylus is in use.

BACKGROUND

Laptop computers are becoming increasingly popular for use by people whomust travel or work in locations where desktop computers are notavailable or practical. Today's laptops are often just as powerful andfeature-rich as many desktops. For example, many laptops now comeequipped with large displays and full size keyboards. Although laptopcomputers are quite useful in appropriate situations, they are ofteneither too large and/or too heavy to be used in remote locations, orwhere space is limited. Furthermore, battery life in laptop computers istypically only about 2-4 hours, further limiting their usefulness inremote locations. Finally, laptop computers may simply be overkill insituations where data entry is all that is required.

One alternative to the laptop computer for mobile data entry is thepersonal digital assistant (PDA). Unlike laptops, which use a computerkeyboard for data entry, PDAs typically employ a stylus that is used towrite on the screen of the PDA. The writing is then captured andprocessed using handwriting recognition software in the PDA.Unfortunately, the screens on most PDAs are relatively small, thuslimiting the amount of text or data that can be entered and viewed atonce. Furthermore, many PDAs require a user to learn and employ aspecial alphabet when inputting handwritten text.

A recent alternative to both the laptop and the PDA for mobile dataentry is what is referred to as a pen based text entry system, or adigital pen. Digital pens typically allow users to write or draw on apad of paper, and capture that writing or drawing to memory within thepen, or in an attached device, such as a PDA. The writing or drawing canthen be transferred at a later time to a conventional computer forprocessing, such as handwriting recognition.

There are a number of ways in which digital pens capture text orwriting. One such way involves the use of a tiny camera located withinthe pen to capture text or data as it is being written. These “camerabased” pens require the use of special paper, which has a series ofmicroscopic dots spaced throughout the paper. As a user writes on thepaper with the pen, a tiny camera near the tip of the pen capturesimages of the dot pattern. A processor in the pen then uses the capturedimages to mathematically determine where the point of the pen was on thepage at the moment the images were captured. By examining the changingdot patterns from image to image, the pen creates a virtual trail ofwhere the pen tip has been. From this data, a record can be made of thepath the pen has taken across the paper.

Unfortunately, these “camera based” digital pens have a number ofdrawbacks. The most significant of these drawbacks is the fact thatspecial “dotted” paper must be used for the pen to function. Thisspecial paper is more expensive than standard paper. Additionally, thisspecial paper is often not readily available. As such, to practicallyuse a camera based digital pen, this special paper must be carried withyou at all times.

One alternative to these camera based digital pens is an ultrasonic typedigital pen. Current ultrasonic type digital pens employ a singleultrasonic transmitter in the pen that transmits an ultrasonic signal toan ultrasonic receiver unit, which is typically clipped or attached to apad of paper. The ultrasonic receiver unit typically includes twoultrasonic receivers for receiving the signal transmitted from the pen.Using simple 2-dimensional triangulation techniques, the location of thetip of the pen on a 2-dimensional plane, such as a sheet of paper, canbe determined.

Unfortunately, these ultrasonic type digital pens are not very accuratein determining the precise location of the pen tip during writing anddrawing. The primary reason for this lack of accuracy relates to theposition of the ultrasonic transmitter in the pen. Due to the size ofthe ultrasonic transmitter and the dimensions of the pen, the ultrasonictransmitter must be located some distance from the pen tip. This is nota problem when the pen is held in a perfectly perpendicular orientationrelative to the paper. Held in this manner, the transmitter is aligneddirectly over the pen tip and, thus, in the same location in the2-dimensional plane as the pen tip. However, when the pen is tilted thetransmitter will no longer be aligned directly over the pen tip. Rather,the transmitter will be located some distance away from the pen tip inthe 2-dimensional space. Since the receiver records the position of thetransmitter, rather than the location of the pen tip, an inaccurate tiplocation will be recorded whenever the pen is tilted, which occursnaturally during writing.

Accordingly, there is a need for pen-based, mobile data capture systemthat accurately records the position of the tip of the pen, rather thanthe location of the transmitter and that does not require the use ofspecial paper.

SUMMARY OF THE INVENTION

Described herein are embodiments of various systems and methods foraccurately determining the location of the tip of an electronic stylus.In accordance with one embodiment, an electronic stylus system includesan electronic stylus and base receiving unit. The electronic stylusincludes two or more ultrasonic transmitters, an electromagnetictransmitter, and a writing tip. The base receiver unit includes two ormore ultrasonic receivers and an electromagnetic receiver. Theultrasonic receivers of the base unit are operable to receive signalstransmitted by the ultrasonic transmitters of the electronic stylus.Similarly, the electromagnetic receiver of the base unit is operable toreceive signals transmitted by the electromagnetic transmitter of thestylus.

Another embodiment relates to a method of locating the position of thetip of an electronic stylus relative to a given reference point. Inaccordance with this embodiment, the locations of two ultrasonictransmitters in the electronic stylus s are first determined relative tothe given reference point. The position of the tip of the stylusrelative to a given reference point is then determined using thedetermined positions of the two ultrasonic transmitters and variousgeometries of the electronic stylus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an electronic stylus system including anelectronic stylus and an electronic stylus base module.

FIG. 2 is a block diagram illustrating various operational components ofthe electronic stylus depicted in FIG. 1.

FIG. 3 is a block diagram illustrating various operational components ofthe electronic stylus base module depicted in FIG. 1.

FIG. 4 is a perspective view of the electronic stylus system of FIG. 1,illustrating various dimensions that are used in determining thelocation of the tip of the electronic stylus depicted in FIG. 1.

FIG. 5 illustrates operations for determining the position of the tip ofthe electronic stylus depicted in FIG. 1.

FIG. 6 is a perspective view of the electronic stylus system of FIG. 1illustrating various parameters and measurements used or calculated indetermining the position of the tip of the electronic stylus depicted inFIG. 1.

FIG. 7 is a perspective view of the electronic stylus system of FIG. 1illustrating various other parameters and measurements used orcalculated in determining the position of the tip of the electronicstylus depicted in FIG. 1.

FIG. 8 is a perspective view of the electronic stylus system of FIG. 1illustrating yet other parameters and measurements used or calculated indetermining the position of the tip of the electronic stylus depicted inFIG. 1.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

The following description sets forth specific embodiments of anelectronic stylus system and method that incorporates elements recitedin the appended claims. The embodiment is described with specificity inorder to meet statutory requirements. However, the description itself isnot intended to limit the scope of this patent. Rather, the inventorshave contemplated that the claimed invention might also be embodied inother ways, to include different elements or combinations of elementssimilar to the ones described in this document, in conjunction withother present or future technologies.

In general, the described embodiments relate to systems and methods foraccurately capturing and recording the location of the tip of a styluswhile the stylus is being used for writing or drawing on conventionalwriting paper, or the like. In accordance with various embodimentsdescribed herein, the stylus, referred to herein as the electronicstylus, includes at least two ultrasonic transmitters and at least oneelectromagnetic transmitter, each of which transmits signals when thestylus is being used for writing or drawing. A base unit, including atleast two ultrasonic receivers and at least one electromagneticreceiver, is then used to receive the ultrasonic and electromagneticsignals transmitted from the electronic pen. These ultrasonic andelectromagnetic signals, together with the fixed distance between theultrasonic transmitters in the pen, the fixed distance between theultrasonic receivers in the base unit, and the location of the stylustip in the pen, are then used to determine the precise position of thestylus tip during writing or drawing operations.

Systems and methods relating to the form and use of electronic styluses,electronic stylus systems, and methods for use thereof, will now bedescribed in detail with reference to a few embodiments, as illustratedin the accompanying drawings. In the following description, numerousspecific details are set forth in order to provide a thoroughunderstanding of these embodiments. It will be apparent, however, to oneskilled in the art that these embodiments may not include, or may bepracticed without, some or all of these specific details. In otherinstances, well known process steps or electronic or mechanical systemshave not been described in detail in order to avoid unnecessarilyobscuring the description of these embodiments.

Turning first to FIG. 1, illustrated therein is an exemplary electronicstylus system 100 in accordance with one embodiment of the presentinvention. The electronic stylus system 100 includes an electronicstylus 102 and an associated electronic stylus base unit 104 (baseunit). The electronic stylus system 100 is shown in an exemplaryoperating environment, including writing material 106, such as a pieceor pad of paper, and a writing surface 108, such as a table top or otherrigid surface.

As shown in FIG. 1, the electronic stylus 102 includes a stylus body 110and a writing or stylus tip 112 located at one end of the stylus body110. In one embodiment, the electronic stylus 102 includes a mechanism114 for holding and dispensing ink or lead (graphite) through the stylustip 112. For example, the mechanism 114 may comprise an ink cartridge orlead dispenser disposed within the stylus body 110 and operablyconnected to the stylus tip 112. In this embodiment, the stylus and ink(lead) may then used in a conventional manner for writing or drawing onthe writing material 106. This conventional usage of the electronicstylus 102 may occur alone, or in conjunction with the transmission ofultrasonic and/or electromagnetic signals, as described in detail below.In other embodiments, the stylus tip 112 may function simply as a stylusfor the electronic capture of writing or drawing strokes, withoutdispensing ink or lead. For example, the electronic stylus may me usedas a replacement for the electromagnetic digitizer in a tablet PC orPDA.

Also included in the electronic stylus 102 are a first ultrasonictransmitter 118 and a second ultrasonic transmitter 120. Preferably,both ultrasonic transmitters 118 and 120 are omni-directional typeultrasonic transmitters, which transmit signals in both horizontal andvertical planes. The ultrasonic transmitters 118 and 120 generate soundwaves or signals above the human ear's audibility limit of about 20 kHz.In one embodiment, the ultrasonic transmitters 118 and 120 transmitsound waves between approximately 30-120 kHz, and more particularly atapproximately 80 kHz. The sound waves transmitted by the ultrasonictransmitters may include, or have encoded therein, various data. Forexample, in one embodiment, the sound waves transmitted by one or moreof the ultrasonic transmitters 118 and/or 120 will include or embodydata identifying the particular electronic stylus that is transmittingthe ultrasonic signals, such as a stylus identification number. In otherembodiments, the sound waves transmitted by the ultrasonic transmitters118 and/or 120 may include or embody other types data.

As shown in FIG. 1, the first and second ultrasonic transmitters 118 and120 are embedded or held in the stylus body 110 at a predetermineddistance from one another. Preferably, the first and second ultrasonictransmitters 118 and 120 and the stylus tip 112 are all aligned ororiented in the stylus body 110 along a single axis 116. Also embeddedor held in the stylus body 110 is an electromagnetic transmitter 122.

In one embodiment, the electromagnetic transmitter 122 is an infrared(IR) transmitter operable to transmit electromagnetic waves having afrequency range above that of microwave, but below the visible spectrum.For clarity, the electromagnetic transmitter 122 will herein after bereferred to as the IR transmitter 122, rather than the electromagnetictransmitter 122. However, it should be understood that theelectromagnetic transmitter 122 may alternatively comprise other typesof electromagnetic transmitters operable to transmit waves havingfrequency ranges other than the IR spectrum. The IR signal transmittedby the IR transmitter 122 may include, or have encoded therein, variousdata. For example, in one embodiment, the signal transmitted by the IRtransmitter 122 will include or embody data identifying the particularelectronic stylus that is transmitting the IR signals, such as an IRtransmitter identification number. Additionally, as described in detailbelow, the signal transmitted by the IR transmitter 122 may includeinformation specifying which of the ultrasonic transmitters 118 or 120is currently transmitting, or is about to transmit. In otherembodiments, the signal transmitted by the IR transmitter 122 mayinclude or embody other data.

In addition to the ultrasonic and IR transmitters, the electronic stylus102 may also include a stylus indicator lamp 124 and a function switch126, both of which are embedded or held in the stylus body 110. In oneembodiment, the stylus indicator lamp 124 comprises a visible LED (lightemitting diode) that functions to indicate a low power condition in theelectronic stylus 102. In other embodiments, the stylus indicator lamp124 may be indicative of other operational functions or states of theelectronic stylus 102.

In one embodiment, the function switch 126 provides a means by which auser of the stylus may select various operational functions of theelectronic stylus system 100. For example, in one embodiment, thefunction switch has two operational states that can be selected by auser of the stylus 102. In this embodiment, the two states may comprisea quiescent state, where no signal is received or detected by the styluscontroller from the function switch, and an indicator state where asignal is received or detected by the stylus controller from thefunction switch. For example, in this embodiment, the second state maybe used to indicate some break or delineation point in the operation ofthe pen, such as a page change. The controller may then be operable tocause the IR transmitter 122 to send an signal having encoded thereinthe state of the function switch.

As previously described, the electronic stylus 102 may be used in aconventional manner using ink or lead (graphite) for writing or drawingon the writing material. As also described, in accordance with thepresent invention, the electronic stylus 102 may be used to transmitultrasonic and IR signals to the base unit 104 via the first ultrasonictransmitter 118, the second ultrasonic transmitter 120, and the IRtransmitter 122.

Turning now to FIG. 2, illustrated therein are various exemplaryoperational components 200 of the electronic stylus 102. As shown inFIG. 2, and in accordance with one embodiment of the electronic stylus102, the operational components 200 may include a stylus microcontroller202, a first ultrasonic driver module 204, a second ultrasonic drivermodule 206, an IR driver module 208, a stylus indicator lamp drivermodule 210, stylus tip contact switch 212, and the function switch 126.As shown, each of the driver modules 204, 206, 208, and 210, as well asthe stylus tip contact switch 212 and the function switch 126, isoperably connected to the microcontroller 202. Additionally, each of thedriver modules is connected to an associated ultrasonic transmitter, IRtransmitter, or indicator lamp. Specifically, the first ultrasonicdriver module 204 is connected to the first ultrasonic transmitter 118,the second ultrasonic driver module 206 is connected to the secondultrasonic transmitter 120, the IR driver module 208 is connected to theIR transmitter 122, and the stylus indicator lamp driver module 210 isconnected to the stylus indicator lamp 124.

In operation, each of the ultrasonic driver modules is operable toreceive a signal from the stylus microcontroller 202 and, in turn,generate high amplitude electrical pulses that drive its associatedultrasonic transmitter. Likewise, the IR driver module 208 is operableto receive a signal from the stylus microcontroller 202 and, in turn,generate appropriate signals to drive the IR transmitter 122. The stylusindicator lamp driver module 210 is operable to receive a signal fromthe stylus microcontroller 202 and, in turn, generate appropriatesignals to drive the stylus indicator lamp 124. In one embodiment, thestylus indicator lamp 124 comprises a visible LED.

The stylus tip contact switch 212 is operable to indicate when thestylus tip 112 is in contact with a firm surface, such as when thestylus 102 is being used to write or draw on the writing material 106.The stylus tip contact switch 212 is positioned in the stylus body 110in such a way as to have a force exerted thereon when the stylus tip 112is in contact with a firm surface. For example, as shown in FIG. 1, inone embodiment, the stylus tip contact switch 212 is positioned at anend of the mechanism 114 that dispenses the ink or lead through thestylus tip 112. Positioned in such a manner, a force will be exerted onthe stylus tip contact switch 212, through the mechanism 114, when thestylus tip 112 is in contact with the writing material 106. In otherembodiments, the stylus tip contact switch 212 may be positioned inother locations within the stylus body, so long as a force will beexerted on the stylus tip contact switch 212 when the stylus tip 112 isin contact with a firm surface.

When a force of a specified magnitude is exerted on the stylus tipcontact switch 212, the stylus tip contact switch 212 will enter anengaged state. In this engaged state, the stylus tip contact switch 212will indicate to the microcontroller 202 that the tip 112 of the stylusis in firm contact with the writing material. As will be appreciated,the pressure switch may either produce a signal indicative of theengaged state or, alternatively, the switch may indicate the engagedstate by allowing the passage of current through the switch. The stylustip contact switch 212 may be of a number of suitable switch or sensortypes. For example, in one embodiment, the stylus tip contact switch 212comprises a zero-movement resistive switch, also known in the art as apressure switch. In another embodiment the contact switch 212 comprisesan analog or digital pressure sensor.

As will be appreciated, the stylus microcontroller 202 operates orcontrols the various components connected thereto in accordance withinstructions embodied in software and/or firmware. More particularly,the software and/or. firmware controlling the microcontroller 202dictates the timing, duration, and type of signals that are sent by themicrocontroller 202 to the first ultrasonic driver module 204, thesecond ultrasonic driver module 206, the IR driver module 208, and thestylus indicator lamp driver module 210. Similarly, the software and/orfirmware dictates how the microcontroller 202 responds to signalsreceived from the stylus tip contact switch 212 and the function switch126.

In accordance with one embodiment, the microcontroller 202 operates tocause the first and second ultrasonic transmitters to alternativelytransmit ultrasonic signals, whenever the stylus tip contact switch 212is in the engaged state, indicating that the stylus tip 112 is incontact with the writing material 106. That is, the microcontroller 202operates to cause one ultrasonic transmitter and then the otherultrasonic transmitter, to transmit signals, in an alternating manner,while the stylus tip contact switch 212 is in the engaged state.Additionally, in accordance with this embodiment, the microcontroller202 operates to cause the IR transmitter to transmit data indicatingwhen, and which of the first or second ultrasonic transmitters ispresently, or is about to, send an ultrasonic signal.

For example, in accordance with one embodiment, the stylusmicrocontroller 202 maintains a waiting state until such time as thestylus tip contact switch 212 enters the engaged state, indicating thatthe stylus tip 112 is in contact with the writing material 106. Upondetermining that the contact switch has entered the engaged state, themicrocontroller 202 then sends a signal to the IR driver module 208,which in turn sends a corresponding signal to the IR LED 122 forbroadcast. In one embodiment, the signal sent to the IR driver module208, and the corresponding signal that is broadcast by the IR LED 122,specifies that the first ultrasonic transmitter 118 is, or is about to,broadcast. Simultaneously, or near simultaneously, with the transmissionby the microcontroller 202 of the signal to the IR driver module 208,the microcontroller 202 sends a signal to the first ultrasonic drivermodule 204. In turn, the first ultrasonic driver module 204 sends acorresponding signal to the first ultrasonic transmitter 118 forbroadcast.

Next, the microcontroller 202 sends a signal to the driver module 208,which in turn sends a corresponding signal to the IR LED 122 forbroadcast. In one embodiment, the signal sent to the IR driver module208, and the corresponding signal that is broadcast by the IR LED 122,specifies that the second ultrasonic transmitter 120 is, or is about to,broadcast. Simultaneously, or near simultaneously, with the transmissionby the microcontroller 202 of the signal to the IR driver module 208,the microcontroller 202 stops sending signals to the first ultrasonicdriver module 118 and sends a signal to the second ultrasonic drivermodule 206. In turn, the second ultrasonic driver module 206 sends acorresponding signal to the second ultrasonic transmitter 120 forbroadcast. This process of alternatively sending signals to the IRmodule 208 and the first ultrasonic driver module 204, and then sendingsignals to the IR module 208 and the second ultrasonic driver module206, continues until the stylus tip contact switch 212 is no longer inthe engaged state. That is, the signals continue to be sent by themicrocontroller until the stylus tip 112 is no longer in sufficientcontact with the writing material 106.

In accordance with another embodiment, the microcontroller 202 operatesto cause the first and second ultrasonic transmitters to alternativelytransmit ultrasonic signals continuously during use of the stylus. Inthis mode of operation, called a “hover” mode, the stylus tip contactswitch 212 would not be used. Rather, the position of the stylus tipwould continue to be tracked regardless of the state of the contactswitch.

Returning now to FIG. 1, as shown therein, the base unit 104 includes abody portion 128 including a first ultrasonic receiver 130 and a secondultrasonic receiver 132. Both ultrasonic receivers 130 and 132 areoperable to detect ultrasonic signals that are transmitted from both thefirst ultrasonic transmitter 118 and the second ultrasonic transmitter120 of the electronic stylus 102. For example, as shown in FIG. 1, thefirst ultrasonic transmitter 118 of the stylus 102 is shown transmittinga first ultrasonic signal 134 that is received by both the firstultrasonic receiver 130 and the second ultrasonic receiver 132.Likewise, the second ultrasonic transmitter 120 of the stylus 102 isshown transmitting a second ultrasonic signal 136 that is received byboth the first ultrasonic receiver 130 and the second ultrasonicreceiver 132.

Also located in the body portion 128 of the base unit 104 is anelectromagnetic receiver 138 that is operable to detect electromagneticsignals 140 from the electromagnetic transmitter 122 of the electronicstylus 102. In one embodiment, the electromagnetic receiver 138 is aninfrared (IR) receiver operable to detect waves having a frequency rangeabove that of microwave but below the visible spectrum. For clarity, theelectromagnetic receiver 138 will herein after be referred to as the IRreceiver 138, rather than the electromagnetic receiver 138. However, itshould be understood that the electromagnetic receiver 138 mayalternatively comprise other types of electromagnetic receivers operableto detect waves transmitted from the electromagnetic transmitter 122having a frequency range other than the IR spectrum.

In addition to the ultrasonic receivers 130 and 132 and IR receiver 138,the base unit 104 also includes a data transfer port 142. As describedin greater detail below, the data transfer port 142 provides a mechanismby which various types of data may be transferred from the base unit toanother computing device or computing process. In accordance with oneembodiment, the data transfer port 142 comprises a physical or wiredconnection port into which a cable may be plugged, so as to physicallyand electrically connect the base unit 104 to another computing device.For example, and without limitation, the data transfer port 142 maycomprise a data communication port conforming to any of a number ofwell-known communication standards and protocols, e.g., parallel,serial, SCSI (small computer system interface), Firewire (IEEE 1394),USB, Ethernet, etc.

In accordance with another embodiment, the data transfer port 142comprises a wireless connection port by which the base unit 104 maycommunicate to or with another computing device or computing process.For example, and without limitation, the data transfer port 142 maycomprise a wireless data communication transmitter or transceiver thatoperates according to the IEEE 802.11x Wireless Networking standards,the “Bluetooth” standard, or according to other standard or proprietarywireless techniques. In accordance with another embodiment, the datatransfer port 142 comprises a removable memory device, such a Flash RAM,memory stick, micro-drive, minidisk or other form of removablenon-volatile storage.

Turning now to FIG. 3, illustrated therein are various exemplaryoperational components 300 of the base unit 104. In accordance with oneembodiment, the operational components 300 include the previouslydescribed first ultrasonic receiver 130, second ultrasonic receiver 132,IR receiver 134, and data transfer port 142. Additionally, theoperational components 300 of the base unit 104 also include a base unitmicrocontroller 302, a first receiver module 304, a second receivermodule 306, and a memory 308. The memory 308 may be a discrete memorydevice or it may be incorporated in the microcontroller 302.

As shown in FIG. 3, the base unit microcontroller 302 is operablyconnected to the first ultrasonic receiver 130 via the first receivermodule 304 and the second ultrasonic receiver 132 via the secondreceiver module 306. Additionally, the base unit microcontroller 302 isoperably connected to a memory 308, the data transfer port 142, and theinfrared receiver 134. In general, the base unit microcontroller 302 isoperable to receive and process ultrasonic signals received at the firstand second receiver modules 304 and 306 and infrared signals received atthe infrared receiver module 134. The manner in which themicrocontroller 302 processes these signals is described in greaterdetail below.

As shown, each of the receiver modules 304 and 306 includes a number ofcomponents for processing ultrasonic signals received from the firstultrasonic receiver 130 and second ultrasonic receiver 132,respectively, before the signals are received by the base unitmicrocontroller 302. For example, in one embodiment, each of thereceiver modules 304 and 306 includes an amplifier 310 and 320, aautomatic gain control (AGC) 312 and 322, a comparator 314 and 324, athreshold signal generator 316 and 326, and a monostable multivibrator318 and 328. Since the components of the receiver modules 304 and 306are identical, the operations of only the first receiver module 304 willbe now described. It will be appreciated that the operations of thesecond receiver module 306 will be identical to the operations of thefirst receiver module 304.

With respect to the first receiver module 304, when an ultrasonic signalis received at the first ultrasonic receiver 130, the first ultrasonicreceiver 130 generates a corresponding first signal 311 that is receivedat the amplifier (AMP1) 310. The amplifier 310 then amplifies the firstsignal 311 by a predetermined magnitude to produce an amplified signal313. Next, the AGC 312 receives the amplified signal 313 and produces acorresponding gain controlled signal 315. Next, the gain controlledsignal 315 is sent to the comparator 314, where it is compared to apredetermined threshold signal 317 provided by the threshold signalgenerator (Th₁) 316. If it is determined at the comparator 314 that gaincontrolled signal 315 is greater than the threshold signal 317, atrigger signal 319 is generated by the comparator 314, which then isreceived by the monostable multivibrator 318. The monostablemultivibrator 318 then produces a signal pulse 321 of fixed duration foreach pulse received at the input to the monostable multivibrator 318.This monostable signal 321 from the monostable multivibrator 318 is thenreceived at the base unit microcontroller 302 for processing.

It should be understood that the specific components and functions ofthe receiver modules 304 and 306, which have been described, areexemplary only. Those skilled in the art will appreciate that theparticular elements and the functions described with respect to thereceiver module 304 may vary. All that is required of the receivermodules 304 and 306 is that they process or condition signals receivedby the first and second ultrasonic receivers 130 and 132 in a mannersuch that the signals may be received and used by the base unitmicrocontroller 302. Stated another way, all that is required of thereceiver modules 304 and 306 is that they provide an appropriateinterface between the first and second ultrasonic receivers 130 and 132and the base unit microcontroller 302.

As previously described, in general, the base unit microcontroller 302is operable to process the received ultrasonic and infrared signals. Inparticular, the microcontroller 302 is operable to determine the timerequired (time value) for an ultrasonic signal to travel from either thefirst ultrasonic transmitter 118 or the second ultrasonic transmitter120 to either the first ultrasonic receiver 130 or the second ultrasonicreceiver 132. As will be appreciated, these time values may be computedand stored in a number of ways. For example, the time values may becomputed as counter values within the microcontroller 302. These timevalues can then be used in time-of-flight (TOF) calculations todetermine the distance between the ultrasonic transmitters andreceivers. TOF calculations use the speed of sound through air tocalculate the distance between the transmitter of an ultrasonic signaland the receiver of that signal.

In accordance with one embodiment, the base unit computes and storesonly the time values. These time values are then stored in the memory308 and, at some later time, transferred to an external computing deviceusing the data transfer port 142. In another embodiment, the base unitmicrocontroller 302 is operable to perform the time-of-flight (TOF)calculations to determine the distances between the ultrasonictransmitters and receivers. These TOF values are then stored in thememory 308 and, at some later time, transferred to an external computingdevice using the data transfer port 142. In yet another embodiment, themicrocontroller 302 is operable to determine the precise position of thetip 112 of the stylus 102, according to methods that will now bediscussed.

It will be appreciated that the precise speed of sound in a givenenvironment may vary. In particular, the speed of sound is dependent on,among other things, the temperature and humidity level of the airthrough which the sound travels. As such, in various embodiments, thestylus 102 or the base unit may include sensors to measure environmentalconditions, such as humidity and/or temperature. These measuredenvironmental conditions may then be used to calculate a more precisevalue for the speed of sound for use in computing the TOF values.

An exemplary method for determining the precise position of the tip 112of the stylus 102, relative to a given point will now be described. Inaccordance with one embodiment, the position of the tip 112 of thestylus 102 is determined relative to a point on the base unit 104. Aspreviously mentioned, this method may be carried out, all or in part, inthe base unit microcontroller 302, or all or in part in an externalcomputing device. This method employs the previously discussed timevalues to establish the distance between the ultrasonic transmitters 118and 120 and the ultrasonic receivers 130 and 132. From this information,together with the fixed distance between the ultrasonic transmitters 118and 120, the fixed distance between the ultrasonic receivers 130 and132, and the distance between the stylus tip and at least one of theultrasonic receivers, a very precise determination can be made ofposition of the tip 112 of the stylus 102, relative to the base unit104.

With respect to computing the distances between the ultrasonictransmitters 118 and 120 and the ultrasonic receivers 130 and 132 usingTOF measurements, it will be appreciated that ultrasonic pulses travelat approximately 340 m/s, depending on temperature and humidityconditions. In addition, the IR signal will be transmittedsimultaneously with the ultrasonic signals, or at a fixed time before orafter the ultrasonic signals. For example, and without limitation, theIR signal may be transmitted between 1 and 5 microseconds before orafter the ultrasonic signals. It will be appreciated that an IR signaltransmitted from the IR transmitter of the electronic stylus will arriveat the IR receiver of the base unit 104 almost instantaneously. That is,relative to the time it takes for an ultrasonic signal to travel betweenthe electronic stylus 102 and the base unit 104, an IR signal travelingbetween the electronic stylus 102 and the base unit 104 will effectivelyis be instantaneous.

With this in mind, the distance between a given ultrasonic transmitterand a given ultrasonic receiver can be determined by first issuing an IRsignal from the IR transmitter 122 to the IR receiver 138.Simultaneously with the transmission of the IR signal, an ultrasonicsignal is sent between the given ultrasonic transmitter and the givenultrasonic receiver. Using the microcontroller, or a simple timingcircuit, the time (t) can then be determined simply by calculating thetime between the receipts of the IR signal and the ultrasonic signal.The distance (d) between the given ultrasonic transmitter and the givenultrasonic receiver can then be calculated using Equation (1):d=v _(s) ×t  Equation (1)In Equation (1), v_(s) equals the speed of sound.

Before proceeding with the discussion of operations for determining thelocation of the stylus tip 112 relative to a given point, it will behelpful to first establish various nomenclature, dimensions, and areference coordinate system, with respect to which the location of thestylus tip 112 may be determined. Turning then to FIG. 4, an origin 402of an xyz-coordinate system is shown located at the first ultrasonicreceiver 130. As shown, the x-axis 404 passes through the firstultrasonic receiver 130 and the second ultrasonic receiver 132. They-axis 406 passes through the first ultrasonic receiver 130 and,together with the x-axis 404, forms a plane substantially parallel withthe writing surface 108 and the writing material 106. The z-axis 408then extends through the origin 402 at the first ultrasonic receiver130, perpendicular to both the x-axis 404 and the y-axis 406. Thoseskilled in the art will appreciate the orientation of the xyz-coordinatesystem shown in FIG. 4 is selected for convenience, and that otherorientations of the xyz-coordinate system may be used. Additionally, theorigin of the coordinate system may be established at a location otherthan at the first ultrasonic receiver. Furthermore, other coordinatesystems, such as non-Cartesian coordinate systems may be used.

Also shown in FIG. 4 are various dimensions that may be used in thedetermination of the location of the stylus tip 112, in accordance withthe present invention. For example, the distance between the first andsecond ultrasonic receivers 130 and 132 is denoted as L 412. Thedistance between the first and second ultrasonic transmitters 118 and120 is denoted as P 414. The distance between first ultrasonictransmitter 118 and the stylus tip 112 is denoted P′ 416.

As previously described, the distance between an ultrasonic receiver andan ultrasonic transmitter may be determined using Equation (1) above.The variables related to these distances are also shown in FIG. 4 asfollows. The distance between the first ultrasonic transmitter 118 andthe first ultrasonic receiver 130 is denoted as d₁₁ 418. The distancebetween the first ultrasonic transmitter 118 and the second ultrasonicreceiver 132 is denoted as d₁₂ 420. The distance between the secondultrasonic transmitter 120 and the first ultrasonic receiver 130 isdenoted as d₂₁ 422. The distance between the second ultrasonictransmitter 120 and the second ultrasonic receiver 132 is denoted as d₂₂424.

Having established a coordinate system and appropriate nomenclature, anexemplary method for determining the location of the stylus tip 112 willnow be described. Turning now to FIG. 5, shown therein is a flow chart500 illustrating exemplary operations that may be performed indetermining the precise location of the stylus tip 112 of the electronicstylus 102, relative to the x-y-z coordinate system described above withrespect to FIG. 4, when the stylus tip is in contact with the writingmaterial 106. FIGS. 6-8 illustrate how the various operationsillustrated in FIG. 5 physically and spatially relate to the electronicstylus system 100.

As previously mentioned, in one embodiment, the operations illustratedin the flow chart 500 may be performed, all or in part, in or by thebase unit microcontroller 302. In other embodiments the operations maybe performed in other microcontroller(s), processor(s), computingdevice(s) or computing systems. The operations may be implemented (1) asa sequence of processor-implemented steps and (2) as interconnectedmachine modules. The implementation is a matter of choice, dependent onperformance and/or application requirements. Accordingly, the operationsmaking up the embodiments described herein are referred to variously asoperations, steps, objects, or modules.

As shown in FIG. 5, at the start of the operations 500, a first locusdetermination operation 502 determines the locus of the first ultrasonictransmitter 118 (the “first locus”). As used herein, a first locus 602(FIG. 6) is a circle of coplanar points equidistant from a center pointP₁ 604, having a radius R₁ 606. As shown in FIG. 6, the first locus 602comprises all of the possible points where the first ultrasonictransmitter 118 could be, given a computed distances d₁₁ 418 between thefirst ultrasonic transmitter 118 and the first ultrasonic receiver 130,and a computed distance d₁₂ 420 between the first ultrasonic transmitter118 and the second ultrasonic receiver 132. The plane on which the firstlocus 602 resides is parallel to the y-axis 406 and perpendicular to thex-axis 404. The first center point P₁ 604 is the point at which theplane defined by the first locus 602 intersects the x-axis 404. As shownin FIG. 6, the first center point P₁ 604 is located at a distance X₁ 608from the origin 402 on the x-axis 404.

As will be appreciated from the foregoing description, the first locus602 may be defined by specifying the distance X₁ 608 and the radius R₁606. As such, in accordance with one embodiment, the first locusdetermination operation 502 comprises determining X₁ 608 and R₁ 606. Inaccordance with one embodiment, the determination of X₁ 608 and R1 606,and hence the determination of the first locus 602, may be made usingEquations (2)-(4):

$\begin{matrix}{L_{1}^{\prime} = {\frac{1}{L}\left( {\left( d_{11} \right)^{2} - \left( d_{12} \right)^{2}} \right)}} & {{Equation}\mspace{14mu}(2)} \\{X_{1} = \frac{L + L_{1}^{\prime}}{2}} & {{Equation}\mspace{14mu}(3)}\end{matrix}$R ₁=√{square root over ((d ₁₁)² −X ₁ ²)}  Equation (4)

As will be appreciated by those skilled in the art, there are a numberof ways to implement Equations (2)-(4) in the software and/or firmware.As such, first locus determination operation 502 is not intended to belimited to any one particular implementation of Equations (2)-(4).

Next, a second locus determination operation 504 determines the locus ofthe second ultrasonic transmitter 120 (the “second locus”). As with thefirst locus 602, the second locus 702 (FIG. 7) is a circle of coplanarpoints equidistant from a center point P₂ 704, having a radius R₂ 706.As shown in FIG. 7, the second locus 702 comprises all of the possiblepoints where the second ultrasonic transmitter 120 could be, given acomputed distances d₂₁ 422 between the second ultrasonic transmitter 120and the first ultrasonic receiver 130, and a computed distance d₂₂ 424between the second ultrasonic transmitter 120 and the second ultrasonicreceiver 132. Again, the plane on which the second locus 702 resides isparallel to the y-axis 406 and perpendicular to the x-axis 404. Thesecond center point P₂ 704 is the point at which the plane defined bythe second locus 702 intersects the x-axis 404. As shown in FIG. 7, thesecond center point P₂ 704 is located at a distance X₂ 708 from theorigin 402 on the x-axis 404.

The second locus 702 may be defined by specifying the distance X₂ 708and the radius R₂ 706. As such, in accordance with one embodiment, thesecond locus determination operation 504 comprises determining X₂ 708and R₂ 706. In accordance with one embodiment, the determination of X₂708 and R₂ 706, and hence the determination of the second locus 702, maymade employing Equations (5)-(7):

$\begin{matrix}{L_{2}^{\prime} = {\frac{1}{L}\left( {\left( d_{21} \right)^{2} - \left( d_{22} \right)^{2}} \right)}} & {{Equation}\mspace{14mu}(5)} \\{X_{2} = \frac{L + L_{2}^{\prime}}{2}} & {{Equation}\mspace{14mu}(6)}\end{matrix}$R ₂=√{square root over ((d ₂₁)²−X₂ ²)}  Equation (7)

As with the Equations (2)-(4), there are a number of ways to implementEquations (5)-(7) in the software and/or firmware. As such, the secondlocus determination operation 504 is not intended to be limited to anyone particular implementation of Equations (5)-(7).

Following the second locus determination operation 504, a determineinclination angle operation 506 determines a first inclination angle θ₁804, and a second inclination angle θ₂ 802. As shown in FIG. 8, theangle θ₂ 804 represents the angle, as measure from the z-axis, at whichthe first ultrasonic transmitter 118 is located on the first locus 602.Similarly, the angle θ₂ 802 represents the angle, as measured from thez-axis, at which the second ultrasonic transmitter 120 is located on thesecond locus 702. In this embodiment, the angles θ₂ and θ₁ arerepresented as angles measured in a clockwise direction from the z-axis408, as viewed from the origin looking in the positive x direction. Thedifference between the second angle θ₂ 802 and the first angle θ₁ 804may be represented as β, where β=θ₂−θ₁.

In accordance with one embodiment, the determine inclination angleoperation 506 may made employing Equations (8)-(14):C=X ₂ −X ₁  Equation (8)

$\begin{matrix}{W = \frac{R_{1}^{2} + R_{2}^{2} + C^{2} - P^{2}}{2 \times R_{1} \times R_{2}}} & {{Equation}\mspace{14mu}(9)}\end{matrix}$β=±arc cos(W)  Equation (10)

In solving Equation (9), it should be noted that P is the distance 808between the first ultrasonic transmitter 118 and the second ultrasonictransmitter 120 of the stylus. It should also be noted that there aretwo solutions to Equation 10. However, only the solutions to Equation(16), described below, should be deemed valid.

Since the stylus tip 112 must be in contact with the writing material106 for the operations 500 to be taking place, the values of θ₂ and θ₁can then be determined using the following equations:

$\begin{matrix}{\lambda = \frac{P^{\prime}}{P}} & {{Equation}\mspace{14mu}(11)}\end{matrix}$

In solving Equation (11), it should be noted that P′ is the distance 806between the first ultrasonic transmitter 118 and the tip 112 of thestylus.

$\begin{matrix}{\phi = {\arctan\left\lbrack \frac{{\lambda\; R_{2}\mspace{11mu}{\cos(\beta)}} - {R_{1}\left( {1 + \lambda} \right)}}{\lambda\; R_{2}\mspace{11mu}{\sin(\beta)}} \right\rbrack}} & {{Equation}\mspace{14mu}(12)}\end{matrix}$

In solving Equation (12) , it should be noted that the variable φ is atemporary variable that is used for calculation purposes only. That is,φ does not correspond directly to a physically measurable angle.

Since there were two solutions for β with respect to Equation (10), βand −β, Equation (12) can likewise be solved for two values of φ, asfollows:

$\begin{matrix}{\phi_{1} = {\arctan\left\lbrack \frac{{\lambda\; R_{2}\mspace{11mu}{\cos(\beta)}} - {R_{1}\left( {1 + \lambda} \right)}}{\lambda\; R_{2}\mspace{11mu}{\sin(\beta)}} \right\rbrack}} & {{Equation}\mspace{14mu}\left( {12\text{-}1} \right)} \\{\phi_{2} = {\arctan\left\lbrack \frac{{\lambda\; R_{2}\mspace{11mu}{\cos\left( {- \beta} \right)}} - {R_{1}\left( {1 + \lambda} \right)}}{\lambda\; R_{2}\mspace{11mu}{\sin\left( {- \beta} \right)}} \right\rbrack}} & {{Equation}\mspace{14mu}\left( {12\text{-}2} \right)}\end{matrix}$

Using Equation (13), we can then solve for θ₁ as follows:θ₁=φ or φ+π  Equation (13)

Since there were two solutions for φ with respect to Equation (12),φ₁and φ₂, Equation (13) can be solved for four values of φ₁, as follows:θ₁₁=φ₁  Equation (13-1)θ₁₂=φ₂  Equation (13-2)θ₁₃=φ₁+π  Equation (13-3)θ₁₄=φ₂+π  Equation (13-1)

Having solved for θ₁, Equation (14) allows us to solve for θ₂.θ₂=θ₁+β  Equation (14)

Since there were four solutions for θ₁ with respect to Equation (12),θ₁₁, θ₁₂, θ₁₃, and θ₁₄, Equation (13) can be solved for four values ofθ₂, two values that correspond to each of positive and negative beta (βand −β).

As will be appreciated by those skilled in the art, there are a numberof ways to implement Equations (8)-(14) in the software and/or firmware.As such, the determine inclination angle operation 506 is not intendedto be limited to any one particular implementation of Equations(8)-(14).

Next, the determine position operation 508 determines the location ofthe stylus tip 112, with respect to the x-axis and y-axis. That is, thedetermine position operation 508 determines an (x, y) coordinate pairspecifying the location of the stylus tip 112 relative to the origin402. In accordance with one embodiment, the determine position operation508 may be made employing Equations (15)-(16) below. More particularly,the value of x may be determined using Equation (15), as follows:x=(1+λ)X ₁ −λX ₂  Equation (15)

There is precisely one value of x that satisfies Equation (15). As willbe appreciated by those skilled in the there are a number of ways toimplement Equation (15) in the software and/or firmware. Additionally,there are a number of ways to combine and substitute the appropriateequations from Equations (2)-(14) to arrive at other forms of Equation(15).

The value of y may be determined using Equation (16), as follows:y=(1+λ)R ₁ sin(θ₁)−λR ₂ sin(θ₂)  Equation (16)

As previously described, there are four possible values for θ₁ and fourpossible values for θ₂. However, in solving for y in Equation (16), onlythe result which produces a non-negative value for y, and for which theangle θ₂ satisfies |θ₂|<90 degrees, will be deemed valid.

As will be appreciated, there are a number of ways to implement andsolve for Equation (16) in the software and/or firmware. Howeverimplemented and solved, only non-negative solution for y will then bedeemed valid. This non-negative solution of y, together with thedetermined value of x, comprise the x-y coordinate of the pen tip.

Following a determine position operation 508, a store position of stylustip operation 510 stores the position (x,y) of the stylus tip in astylus tip position file. In the case where each of the operations 500is performed in the base unit 104, these positions will be stored in thememory 308 of the base unit 104.

In an embodiment where some or all of the operations 500 are performedin a computing device or process outside of the base unit 104, thesepositions will be stored in memory that is accessible by the computingdevice or process located outside of the base unit 104. As will beappreciated, after the operations 500 have been performed a number oftimes, the stylus tip position file will contain a number of pointsspecifying the location of the stylus tip 112 over a given time period.

In an embodiment where each of the operations 500 are performed in thebase unit 104, and where the positions are stored in the memory 308 ofthe base unit 104, a transmit position of stylus tip operation 512 maybe used to transmit the positions of the stylus tip, as recorded in thestylus tip position file, to a computing device or computing processexternal to the base unit 104. In one embodiment, the transmission ofthe stylus tip position file to a computing device or computing processwill occur at some time after the file has been created. For example,the stylus tip positions may be stored while the electronic stylussystem is being used at a location remote from the computing device orprocess. Then, when convenient, the stylus tip position file may betransmitted to, the computing device or process. Alternatively, thetransmission of the stylus tip position file to the computing device orcomputing process may occur in real time. As previously described, thestylus tip position file may be transferred to the external computingdevice or computing process via the data transfer port 142. As alsopreviously noted, the data transfer port 142 may comprise a physical orwired connection port, a wireless connection port, or a removablenon-volatile memory device.

In an embodiment where each of the operations 500 are performed in acomputing device or computing process external to the base unit 104,either the measured distance between the ultrasonic transmitters andultrasonic receivers, or the measured times required for the ultrasonicsignals to travel between the ultrasonic transmitters and ultrasonicreceivers, will be stored in the memory. These distance or timemeasurements, or a series of these distance or time measurements, wouldthen be transferred to the external computing device or computingprocess for processing according to the operations 500.

Although various embodiments set forth herein have been described inlanguage specific to structural features and/or methodological steps, itis to be understood that the invention defined in the appended claims isnot necessarily limited to the specific features or steps described.Rather, the specific features and steps are disclosed as representativeforms of implementing the claimed invention.

1. A method comprising: determining a locus of positions of a firstultrasonic transmitter in an electronic stylus relative to a givenreference point; determining a locus of positions of a second ultrasonictransmitter in the electronic stylus relative to the given referencepoint; and determining a position of a writing tip in the electronicstylus relative to the given reference point using the determined locusof positions of the first ultrasonic transmitter, determined locus ofpositions of the second ultrasonic transmitter, and relative positionsof the first ultrasonic transmitter, the second ultrasonic transmitter,and the writing tip within the stylus, wherein determining the locus ofpositions of the first ultrasonic transmitter comprises: emitting anelectromagnetic signal from an electromagnetic transmitter in theelectronic stylus; receiving the electromagnetic signal at a firstelectromagnetic receiver; emitting an ultrasonic signal from the firstultrasonic transmitter; receiving the ultrasonic signal at a firstultrasonic receiver, wherein the electromagnetic signal specifies fromwhich of the first and second ultrasonic transmitters the receivedultrasonic signal was emitted; and calculating a distance (d₁₁) betweenthe first ultrasonic transmitter and the first ultrasonic receiver basedon a difference in time between when the electromagnetic signal wasreceived and when the ultrasonic signal was received, wherein the firstultrasonic transmitter and the second ultrasonic transmitter alternatelytransmit ultrasonic signals indicating that the writing tip is incontact with a writing material.
 2. The method of claim 1, wherein thegiven reference point is located in an electronic stylus base receiverunit including the first ultrasonic receiver and a second ultrasonicreceiver.
 3. The method of claim 2, wherein the given reference point islocated at the first ultrasonic receiver of the base receiver unit. 4.The method of claim 3, wherein the locus of positions of the firstultrasonic receiver, relative to the reference point, is furtherdetermined by calculating a distance (d₁₂) between the first ultrasonictransmitter and the second ultrasonic receiver.
 5. The method of claim4, wherein the locus of positions of the second ultrasonic receiver isdetermined by calculating a distance (d₂₁) between the second ultrasonictransmitter and the first ultrasonic receiver and calculating a distance(d₂₂) between the second ultrasonic transmitter and the secondultrasonic receiver.
 6. The method of claim 2, wherein the firstultrasonic receiver and the second ultrasonic receiver of the base unitare located at a fixed distance from one another in the base unit, andwherein the fixed distance between the first and second ultrasonicreceivers is used in determining the location of the tip of the stylusrelative to the given reference point.
 7. A method of determining aposition of a tip of a stylus relative to a coordinate system, thestylus having a first ultrasonic transmitter at a first fixed locationin the stylus and a second ultrasonic transmitter at a second fixedlocation in the stylus, comprising: determining a first locus ofpositions for the first ultrasonic transmitter relative to thecoordinate system; determining a second locus of positions for thesecond ultrasonic transmitter relative to the coordinate system;determining a first inclination angle of the first ultrasonictransmitter; determining a second inclination angle of the secondultrasonic transmitter; and determining the location of the tip of thestylus relative to the coordinate system using the first locus ofpositions, the second locus of positions, the first inclination angle,the second inclination angle, and the locations of the first ultrasonicreceiver, the second ultrasonic receiver, and the tip of stylus relativeto one another, wherein determining the first locus of positions for thefirst ultrasonic transmitter comprises: emitting an electromagneticsignal from an electromagnetic transmitter in the stylus; receiving theelectromagnetic signal at a first electromagnetic receiver; emitting anultrasonic signal from the first ultrasonic transmitter; receiving theultrasonic signal at a first ultrasonic receiver, wherein theelectromagnetic signal specifies from which of the first and secondultrasonic transmitters the received ultrasonic signal was emitted; andcalculating a distance between the first ultrasonic transmitter and thefirst ultrasonic receiver based on a difference in time between when theelectromagnetic signal was received and when the ultrasonic signal wasreceived, wherein the first ultrasonic transmitter and the secondultrasonic transmitter alternately transmit ultrasonic signalsindicating that the tip of the stylus is in contact with a writingmaterial.
 8. The method of claim 7, further comprising storing thedetermined location of the tip of the stylus.
 9. The method of claim 8,further comprising transmitting the determined location of the tip ofthe stylus to a computing device.
 10. A method comprising determining adistance between a first ultrasonic transmitter in an electronic pen anda first ultrasonic receiver in a base unit; determining a distancebetween the first ultrasonic transmitter in the electronic pen and asecond ultrasonic receiver in the base unit; determining a distancebetween a second ultrasonic transmitter in the electronic pen and thefirst ultrasonic receiver in the base unit; determining a distancebetween the second ultrasonic transmitter in the electronic pen and thesecond ultrasonic receiver in a base unit; and storing each of thedetermined distances in a non-volatile memory, wherein determining thedistance between the first ultrasonic transmitter and the firstultrasonic receiver comprises: emitting an electromagnetic signal froman electromagnetic transmitter in the electronic pen; receiving theelectromagnetic signal at a first electromagnetic receiver in the baseunit; emitting an ultrasonic signal from the first ultrasonictransmitter; receiving the ultrasonic signal at the first ultrasonicreceiver, wherein the electromagnetic signal specifies from which of thefirst and second ultrasonic transmitters the received ultrasonic signalwas emitted; and calculating the distance between the first ultrasonictransmitter and the first ultrasonic receiver based on a difference intime between when the electromagnetic signal was received and when theultrasonic signal was received, wherein the first ultrasonic transmitterand the second ultrasonic transmitter alternately transmit ultrasonicsignals indicating that a tip of the electronic pen is in contact with awriting material.
 11. The method of claim 10, wherein the distancebetween the first ultrasonic transmitter and the first ultrasonicreceiver is determined using a microcontroller in the base unit.
 12. Themethod of claim 11, wherein the distance between the first ultrasonictransmitter and the first ultrasonic receiver is stored in anon-volatile memory in the base unit.
 13. The method of claim 12,wherein the stored distance between the first ultrasonic transmitter andthe first ultrasonic receiver are transmitted to a computing device. 14.A method of determining a position of the tip of a stylus relative to acoordinate system, the stylus having a first ultrasonic transmitterlocated at a first fixed location in the stylus and a second ultrasonictransmitter located at a second fixed location in the stylus,comprising: receiving a distance between a first ultrasonic transmitterin an electronic stylus and a first ultrasonic receiver in a base unit;receiving a distance between the first ultrasonic transmitter in theelectronic stylus and a second ultrasonic receiver in the base unit;receiving a distance between a second ultrasonic transmitter in theelectronic stylus and the first ultrasonic receiver in the base unit;receiving a distance between the second ultrasonic transmitter in theelectronic stylus and the second ultrasonic receiver in a base unit; anddetermining the position of the tip of the stylus relative to thecoordinate system using the received distances, wherein the distancebetween the first ultrasonic transmitter and the first ultrasonicreceiver is determined by: emitting an electromagnetic signal from anelectromagnetic transmitter in the electronic stylus; receiving theelectromagnetic signal at a first electromagnetic receiver in the baseunit; emitting an ultrasonic signal from the first ultrasonictransmitter; receiving the ultrasonic signal at the first ultrasonicreceiver, wherein the electromagnetic signal specifies from which of thefirst and second ultrasonic transmitters the received ultrasonic signalwas emitted; and calculating the distance between the first ultrasonictransmitter and the first ultrasonic receiver based on a difference intime between when the electromagnetic signal was received and when theultrasonic signal was received, wherein the first ultrasonic transmitterand the second ultrasonic transmitter alternately transmit ultrasonicsignals indicating that the tip of the stylus is in contact with awriting material.
 15. A method comprising: receiving distances between afirst ultrasonic transmitter located in a stylus and each of twoultrasonic receivers located in a base unit; receiving distances betweena second ultrasonic transmitter located in the stylus and each of thetwo ultrasonic receivers; and determining a location of a tip of thestylus relative to the base unit using the received distances, a fixeddistance between the two ultrasonic transmitters, a fixed distancebetween the two ultrasonic receivers, and a location of the tip of thestylus relative to one of the ultrasonic transmitters, wherein thedistance between the first ultrasonic transmitter and one of the twoultrasonic receivers is determined by: emitting an electromagneticsignal from an electromagnetic transmitter in the stylus; receiving theelectromagnetic signal at a first electromagnetic receiver in the baseunit; emitting an ultrasonic signal from the first ultrasonictransmitter; receiving the ultrasonic signal at the one of the twoultrasonic receivers, wherein the electromagnetic signal specifies fromwhich of the first and second ultrasonic transmitters the receivedultrasonic signal was emitted; and calculating the distance between thefirst ultrasonic transmitter and the one of the two ultrasonic receiversbased on a difference in time between when the electromagnetic signalwas received and when the ultrasonic signal was received, wherein thefirst ultrasonic transmitter and the second ultrasonic transmitteralternately transmit ultrasonic signals indicating that the tip of thestylus is in contact with a writing material.
 16. A method comprising:receiving a first time values indicative of a time required for anultrasonic signal to travel between a first ultrasonic transmitter in anelectronic pen and a first ultrasonic receiver in a base unit; receivinga second time value indicative of a time required for an ultrasonicsignal to travel between the first ultrasonic transmitter and a secondultrasonic receiver in the base unit; receiving a third time valueindicative of a time required for an ultrasonic signal to travel betweena second ultrasonic transmitter in the electronic pen and the firstultrasonic receiver; receiving a fourth time value indicative of a timerequired for an ultrasonic signal to travel between the secondultrasonic transmitter and the second ultrasonic receiver; anddetermining a location of a tip of the electronic pen relative to thebase unit using each of the time values, a fixed distance between thetwo ultrasonic transmitters, a fixed distance between the two ultrasonicreceivers, and a location of the tip of the electronic pen relative toone of the ultrasonic transmitters, wherein the time values indicativeof the time required for the ultrasonic signal to travel between thefirst ultrasonic transmitter and the first ultrasonic receiver isdetermined by: emitting an electromagnetic signal from anelectromagnetic transmitter in the electronic pen; receiving theelectromagnetic signal at a first electromagnetic receiver in the baseunit; emitting an ultrasonic signal from the first ultrasonictransmitter; receiving the ultrasonic signal at the first ultrasonicreceiver, wherein the electromagnetic signal specifies from which of thefirst and second ultrasonic transmitters the received ultrasonic signalwas emitted; and calculating the time required for the ultrasonic signalto travel between the first ultrasonic transmitter and the firstultrasonic receiver based on a difference in time between when theelectromagnetic signal was received and when the ultrasonic signal wasreceived, wherein the first ultrasonic transmitter and the secondultrasonic transmitter alternately transmit ultrasonic signalsindicating that the tip of the electronic pen is in contact with awriting material.